halo 3 question?

[woody woood]

how do you unlock more armor on halo 3 and helmet for your character?i have a few of them but i dont know how to get the horn helmt and some armor
?please
does it requres beatinig it on legendary mode to?

 

[abstractfallacy2003]

You just have to get all the achievements? I need 3 more and I will have everything unlocked.

gamertag= Pious Maximus